The present invention relates in general to heating tools for thermally treating objects and/or for connecting objects together by welding, soldering, or gluing. The heating tools may either be moved to the objects to be treated or mounted in a fixed position, with the objects to be treated being brought to the heated tool.
Thermal energy is either supplied to the objects to be treated (e.g. to connect them to each other), by radiant heating from heated surfaces of the tool, or by direct thermal contact between the heated tool and the object. A controlled force is usually applied against the object by the heated tool. An additional thermal conductive material may be used to increase heat flow from the heated tool to the object. Flux, for example, may be provided for increasing the transfer of heat and for reducing oxidation during the thermal process.
Such tools may supply heat either continuously or during selected time periods only. Heat may be applied during heating pulses which may be activated or controlled by various external conditions, for example, by forcing the heated tool against the object to be heated or connected.
Various sources of heat are known for heating the heated tool to a variety of selected temperatures. Hot gases and/or electric current for example may be used for heating specified heat supplying areas or surfaces of the tool.
For the present invention, it is preferable to utilize the flow of electrical current through a heated tool for heating the tool. This permits controlled pulse-heating featuring fast heating rates as well as precise temperature control at various areas of the tool.
A known generally U-shaped heated tool is disclosed in German Patent reference 2,848,519. This heated tool is mounted to holders either using screws or by welding. This keeps the heat supplying areas of the tool in position and further is used to provide electrical connectors for feeding electrical heating current to the tool. Such tools can be manufactured using sheet metal designs (see German Pat. Nos. 3,144,048.7, 3,137,859.5 and 3,137,860.9 to the present inventor). These heated tools can also be machined from solid materials.
When transferring thermal energy to one or more objects using one or more heated tools, excess heat and relatively long periods of time are sometimes needed, particularly when the objects are made of materials having high thermal loads.
The use of force-controlled thermal contact for transferring thermal energy from a tool to a part must also be accurately controlled. For this reason force-controlled thermal contact has limited applications. Objects having high thermal conductivity, for example objects made of metal oxides, require different procedures. Continuous preheating for example may be necessary for such applications. A heating tool is simply set at a much higher temperature to compensate for thermal loads. This has limitation however in that excess heat may be damaging to some materials.
FIG. 1 illustrates a typical configuration for connecting a first object 4, in this case a ribbon or wire extending from an electronic component 6, to a second object 5 in this case a metallization layer on a substrate 5. To this end, a heated tool 1 is applied with a force in the direction of arrows 3 against the first object 4 to heat it and press it against the second object 5. The force and heat of the heated tool 1 is intended to melt a solder coating on the first object 4 as well as a solder coating on the second object 5 in order to melt the solders and connect the objects to each other. The intention is to utilize force in the direction of arrows 3 to simultaneously melt solders to join the object.
This process becomes more critical and difficult however if substrate 5 is made of material having a high thermal load. The temperature at object 4 will lose heat through substrate 5 and therefore be cooler than heated tool 1. The wire or ribbon of object 4 represents an additional thermal resistance between heated tool 1 and object 4. In order to melt the solder below the object therefore heated tool 1 must be brought to a much higher temperature. This increases the chances of damaging sensitive nearby parts, such as the electronic component 6. This also requires excessive heat and time to make the connection.
The process of transferring thermal energy becomes more critical if high thermal loads are in contact with the heat supplying area la of the heated tool 1.